The present invention relates to a method of preventing distortion during cooling of a heated workpiece having a cavity therein.
During the next decade it is expected that a significant market will exist for very large cylindrical forgings used in coal gasification reactors and liquefaction pressure vessels. These vessels are of very large diameter and length requiring a series of the cylinders to be welded or otherwise fastened in end-to-end relationship. The cylinders have an internal diameter and length both of about twelve-feet (12') in each direction, and wall thickness of about eight-inches (8") or thicker. After forging, the normal procedure is to rough machine the inside and outside surfaces of the cylinder, austenitize, quench and temper, and then machine all over to final size. It will be apparent that the dimensional and concentricity specification tolerance must be close in view of the subsequent end-to-end welding operation for construction of the vessels. Significant savings could be realized if it were possible to eliminate the final machining operation. This may be possible if distortion during the tempering operation could be prevented. Elimination of the final machining operation would enable the manufacturer to leave far less stock on the forged product resulting in a significant increase in yield, as well as a greater production rate for a given number of large turning machines.
In the past a large single circular-shaped plate has been used for insertion into the cavity of a heated forging for preventing distortion of the forging during cooling. This practice has been used on forgings in which it was only necessary to control distortion at a recess groove located approximately ten-inches (10") from the bottom face of the forging. It was not necessary to control distortion over the full length of the total cavity in these forgings because of their greater wall thickness and shorter height than the cylinders to which the method of the present invention is directed. Prior to insertion, the plate was carefully machined to a diameter at or only very slightly less than the desired cold size of the cavity in the forging. Upon cooling, the forging contracts and may place the plate in compression to some degree. Removal of the plate sometimes is very difficult causing it to become bent and rendering it unsuitable for further use.
Expandable arbor devices are known for various purposes on relatively small size workpieces. For example, U.S. Pat. No. 2,616,462, Haddican, U.S. Pat. No. 3,742,186, Finkel et al, and U.S. Pat. No. 3,792,856, Hernandez illustrate this type of apparatus. However, none of the references expressly or impliedly suggest that the devices shown therein may be used for preventing distortion of very large cylinders during cooling. Moreover, the devices are not suitable for such use since setting the arbor at an initial expanded position of precise dimension is not possible with the configurations shown.
It is therefore a primary object of the present invention to provide a method of preventing distortion of large workpieces during cooling which overcomes the above-mentioned problems and disadvantages of the prior method.